1. Field of the Invention
The invention relates to electrical devices that are used to supply X-ray tubes.
An X-ray tube comprises a cathode having a filament that emits an electron beam towards an anode or anticathode. The anode is constituted by a material such as tungsten or molybdenum which emits X-rays when it is bombarded by the electron beam coming from the cathode. To obtain a high-energy electron beam, the electrons are accelerated by a high electrical field created between the cathode and the anode. To this end, the anode is taken to a positive potential of several tens of kilovolts with respect to the cathode. This potential may exceed hundred kilovolts and reach 140 kilovolts and more.
2. Description of the Prior Art
Such high voltages are given by so-called high-voltage power devices which, as can be seen in FIG. I, include a transformer 10 that is connected to voltage rectifier-doubler circuits 11. More specifically, the transformer 10 comprises a single primary winding 12 to which there is applied an AC voltage and a secondary circuit 13 which is connected to the voltage rectifier-doubler circuits il. In a standard way, each voltage rectifier-doubler circuit 11 consists of a secondary winding 14, two diodes D1 and D2 and two capacitors C1 and C2 which are connected to each other according to the diagram of FIG. 1. Each voltage rectifier-doubler circuit is connected to the next one in such a way that the output voltages of these rectifier-doubler circuits get added up, thus making it possible to obtain a very high voltage at the last doubling circuit of the assembly.
More specifically, the transformer 10 comprises a primary winding 12 and twelve secondary windings S1 to S12 of which only the elements S1, S5, S6 and S12 have been shown. Similarly, it has 24 identical rectifier diodes D1 to D24 of which only the elements D1, D2, D3, D12, D13, D14, D22, D23, D24 have been shown. Clearly, each diode may be replaced by several diodes in series to take account of the reverse voltage of the diodes.
The transformer 10 also includes 24 filtering capacitors C1 to C24, of which only the elements C1, C2, C3 ... C12, C13, C14 ... C23, C24 have been shown.
Each secondary winding SI to S12 has two output terminals. All the output terminals bear the references B1 to B24. Only the terminals B1, B2, B3 ... B5, B6, B7, B8 ... B23, B24 have been shown.
In FIG. 1, the common point of the capacitor Cl and of the diode D1 constitutes the high voltage (HT) output terminal 46 through a resistor R while the common point of the capacitor C24 and of the diode D24 constitutes the ground output terminal with which a discharge gap 9 is associated.
To measure the amplitude of the high voltage, the high-voltage output terminal 46 is connected to a measuring device (not shown) connected to the point M by means of a resistor R and a variable capacitor C. The point M is connected to the ground by a discharge gap 8.
In one typical exemplary embodiment, each rectifier-doubler circuit has an output voltage of six kilovolts in such a way that, at output of the twelfth rectifier-doubler circuit, the voltage is equal to 72 kilovolts.
It will be noted that, to obtain a potential difference of the order of 140 kilovolts between the cathode and the anode of an X-ray tube, it is enough to connect the cathode to a negative potential of 70 kilovolts with respect to the ground and the anode to a positive potential of 70 kilovolts with respect to the ground. To this effect, two supply devices, identical to that of FIG. 1 are used.
It will be understood that the making of a high-voltage power device according to the circuit of FIG. 1 leads to problems of insulation that are often resolved by moving the conductors with greatly different potentials away from one another and by interposing, between them, an insulating medium such as oil which acts, at the same time, as a cooling liquid. This results then in large-sized devices which take up a lot of space.
Furthermore, the X-ray tubes are increasingly being used in pulsed mode according to increasingly high frequencies. In the circuit of FIG. 1, this means that the primary winding is supplied by an AC voltage with a high frequency, of the order of several tens of kilohertz. Under these new conditions of operation, the performance characteristics of the circuit of FIG. 1 are limited by the parasitic capacitance and self-inductance of the conductors and windings of the transformer, the values of which are difficult to ascertain and compensate for.
In the U.S. Pat. No. 5,003,452, the Applicant for the present application has described a power device in which the relative positions of the different elements lead to minimize the parasitic capacitance and self-inductance values and contribute to reducing the space occupied by the unit while at the same time facilitating the manufacturing process.
Furthermore, through the making of the secondary circuit in the form of concentric windings, only the parasitic capacitance between the first secondary winding and the ground has an influence, for the other parasitic capacitances between the secondary windings are not considered because they are at an AC voltage.
Finally, to limit the lengths of the connection conductors that link the output terminals B1 to B24 of the secondary windings S1 to S12, on the one hand to the diodes D1 to D24 and on the other hand to the capacitors C1 to C24, the invention described in the above-mentioned patent provides, firstly, for making secondary windings, of which the similar odd-order output terminals B1, B3 ... B23 are positioned on a first lateral side of the windings while the even-order output terminals B2, B4 ... B24 are positioned on the other or second lateral side of the secondary windings. There is then provision for grouping the diodes D1 to D24 together on a same support which is positioned on the same side as the output terminals B1, B3 ... B23 of the secondary windings, and for making their connections, firstly, to the diodes D1 to D24 on the first lateral side of the secondary windings and, secondly, to the output terminals B2, B4 ... B24 on the second lateral side of the secondary windings.
Given the power values to be used, the high-voltage power device described in the above-mentioned U.S. patent is placed in a chamber filled with an insulating coolant and the assembly constitutes what is called a high-voltage unit or high voltage pack.
To cool a supply device such as this, comprising a primary winding, secondary windings and other components such as diodes, a substantial volume of cooling liquid, equal to about 15 to 20 liters, is needed. This volume entails a fairly bulky high-voltage pack.
To reduce the space occupied by a high-voltage pack such as this, it has been proposed to position the primary circuit and the magnetic circuit outside the chamber containing the cooling liquid. Said chamber then contains only the secondary circuits and associated components which are taken to high voltages of several kilovolts while the primary circuit is at a relatively low voltage of some hundreds of volts.
A high-voltage pack such as this has been described in the U.S. Pat. No. 5,060,253
A high-voltage pack such as this, apart from its reduction in volume, has satisfactory electrical characteristics for most of the current applications and can thus be used to achieve high voltages of more than 100 kilovolts.
However, since the trend is towards the application of even higher values of high voltages to the X-ray tube and towards increasing the power delivered by the X-ray tubes, a high-voltage pack such as this has certain limitations due to the heating of the secondary circuits and of the rectifier diodes.
Furthermore, the magnetic circuits that may be used are of the type resulting from the combination of a first C-shaped or horseshoe-shaped circuit and a second I-shaped circuit which closes the first circuit. Now, the maximum surface area of the window of passage of such magnetic circuits is limited: this limits the surface area available for the windings.
Finally, if the secondary circuits are connected so as to apply a positive high voltage to the anode and a negative high voltage to the cathode, of the order of 75 kilovolts each, it is difficult if not impossible to obtain perfect symmetry between the two high voltages. Indeed, since the midpoint corresponds to one of the secondary windings, the negative high voltage will correspond, for example, to windings close to the magnetic circuit while the positive high voltage will correspond to windings remote from the magnetic circuit. In this arrangement, the result is that the positive high-voltage windings are subjected to a magnetic flux that is weaker than that undergone by the negative high-voltage windings. It is obviously possible to correct this dissymmetry by providing for a smaller number of turns for the windings of the negative high voltage (internal layers) than for the windings of the positive high voltage (external layers). Corrections such as these complicate the making of a high-voltage pack such as this with symmetrical high voltages without thereby in any way achieving perfect symmetry.
An object of the present invention, therefore, is to make a high-voltage power device and, more particularly, a high-voltage power pack that can provide power at least double that of the power devices and packs described in the above-mentioned patents.
Another object of the present invention is to make a high-voltage power pack that can give perfectly symmetrical high voltages.
These different objects are achieved by the use of two separate secondary circuits that are coupled to at least one primary circuit by means of a magnetic circuit, the passageway window of which is doubled in area by the combination of two C-shaped or horseshoe-shaped magnetic circuits that are positioned so as to face each other.
The fact of using two separate secondary circuits makes it possible to double the number of diodes, thus promoting improved voltage strength when the two secondary circuits are series-connected or an increase in the current when the two secondary circuits are parallel-connected.
Should the two secondary circuits be identical, it is possible to obtain high voltages that are perfectly symmetrical with respect to the ground by connecting the secondary circuits in series and by connecting the connection point to the ground.